bug#66212: 30.0.50; C-x v v and vc-diff with fossil-scm

[Alfred M. Szmidt]

   > From: "Alfred M. Szmidt" <ams@gnu.org>
   > Date: Tue, 26 Sep 2023 08:25:01 -0400
   > 
   > 
   > When trying to use C-x v v in a vc-diff buffer, and when the version
   > control system is Fossil (this is using GNU patch):
   > 
   > Checking in foo...
   > user-error: Patch failed: can't find file to patch at input line 5
   > Perhaps you used the wrong -p or --strip option?
   > The text leading up to this was:
   > --------------------------
   > |Index: foo
   > |==================================================================
   > |--- foo
   > |+++ foo
   > --------------------------
   > File to patch: 
   > Skip this patch? [y] 
   > Skipping patch.
   > 1 out of 1 hunk ignored
   > 
   > 
   > To reproduce, make a empty Fossil repostiroy, add and commit a file,
   > add a line, C-x v =, C-x v v.  vc-fossil has to be installed.

   Crystal ball says that vc-fossil (which is not part of Emacs, AFAICT)
   either doesn't instruct Fossil to produce root-relative file names in
   the diffs (IOW, "foo" above should actually ne "some/directory/foo"),

The file foo was in ./ -- so I think the above is correct.

   or doesn't set the default-directory of the vc-diff buffer to the
   directory of the files as named in the diffs.

I'll check that.